Temperature-independent voltage references are used in many different applications. For example, they can help ensure stability of oscillators, digital-to-analog converters (DACs) and analog-to-digital converters (ADCs), phase-locked loops (PLLs), linear regulators, DC-DC converters, RF circuits, and body-bias generators. Many prior-art voltage reference designs rely on a combination of elements with differing temperature characteristics. The combination typically results in a reference voltage equal to the semiconductor bandgap voltage (approximately 1.2V for silicon). This voltage can be multiplied to produce higher-valued references.
As microelectronic circuit processing techniques and material purities improve, smaller and more power-efficient circuits can be constructed. However, these smaller circuits often have correspondingly smaller process maximum voltages (“Vmax”)—that is, voltages above which the circuit elements will be damaged. In some circuits, the process maximum voltage can be less than the semiconductor bandgap voltage (approximately 1.2V for silicon). Voltage references that can produce a stable, temperature-independent reference of less than the semiconductor bandgap voltage may be useful in combination with these circuits.
FIG. 1 shows a prior-art voltage reference as taught in A Precision Reference Voltage Source by Karel E. Kuijk (IEEE Journal of Solid State Circuits, Vol. SC-8, No. 3, June 1973). Current I1 through diode 110 and current I2 through diode 120 and resistor 130 produce voltages V1 and V2, respectively; op-amp 140 produces a feedback signal VR that is largely independent of temperature, and substantially equal to the semiconductor bandgap voltage of about 1.2V for silicon. Diodes 110 and 120 may be implemented as the base-emitter junctions of bipolar transistors.
FIG. 2 shows another prior-art voltage reference as taught in A CMOS Bandgap Reference Circuit with Sub-1-V Operation by Hironori Banba et al. (IEEE Journal of Solid-State Circuits, Vol. 34, No. 5, May 1999). This circuit can produce an arbitrarily low reference by adjusting resistor 240, but it has several drawbacks compared to Kuijk's reference. First, it requires three matched current sources (MOSFETs 210, 220 and 230) that, in the deep submicron technologies of modem circuits, are difficult to manufacture due to gate leakage and threshold voltage variation. Second, even if three identical MOSFETs could be made, drain-source voltages across the devices are not equal over a wide temperature range. This causes current mismatch due to a finite drain output impedance. These difficulties can cause a reference variation of as much as 1%. Third, the output of the circuit cannot be loaded—drawing even a small current from the reference at 250 will change the voltage. Fourth, the circuit cannot be used in a shunt configuration (explained below) because it requires a supply voltage 260 that is larger than VR.